Global Navigation Satellite Systems (GNSS)'s now come in several forms. The United States government maintains the global positioning system (GPS) of earth orbiting GPS positioning satellites that broadcast GPS signals in several formats. The European Space Agency is deploying a Galileo constellation of positioning satellites. And, Russia has been developing its global navigation satellite system (GLONASS) of positioning satellites for many years. The GNSS satellites provide signals having location-determination information and code timing that can be received, measured and decoded by a GNSS receiver for determining a geographical location of the receiver and an accurate GNSS-based clock time.
The acquisition process for finding signal power in a GNSS signal involves correlating a pseudorandom noise (PRN) code carried on the incoming GNSS signal broadcast by the GNSS satellite against a locally generated PRN code replica. The local code replica is correlated with the incoming code at successive code phase offsets until a code phase offset is found that shows signal power. This process is known as a code search.
When signal power is found, the GNSS receiver uses inversions of the code for determining data bit timing. The data bit timing is used for monitoring the GNSS data bits of the incoming GNSS signal until a GNSS clock time for a time-of-transmission is decoded from the data bits. The time-of-transmission is used with orbital ephemeris parameters for a GNSS satellite for calculating the satellite's location-in-space. The locations-in-space for several GNSS satellites are used with the code phase offsets and the data bit timings for providing pseudoranges between the GNSS receiver and the satellites. These ranges are termed “pseudo” because they depend upon the local replica clocking offset. The GNSS receiver performs arithmetic operations on the locations-in-space and the pseudoranges for resolving the replica clocking offset and the location of the receiver. The resolution of the clocking offset and times-of-transmission are used for determining the GNSS clock time.
Conventional GNSS receivers use four GNSS satellites for resolving the four GNSS unknowns for the three dimensions of the geographical location of the GNSS receiver and the fourth dimension for the clocking offset. It is also conventional for GNSS receivers to use fewer than four satellites when other positioning information, such as altitude, inertial motion or map matching, is available to substitute for the positioning information of a pseudorange; and to use more than four satellites for overdetermining the four unknowns.
The GPS C/A code signal data bits have frames having time periods of thirty seconds. The frames are segmented into five subframes of six seconds each. The time-of-transmission for the GPS signal is encoded in the GPS data bits in a Z-count at six second intervals near the beginnings of the subframes. Unfortunately, this means that about six seconds of GPS data bits must be observed in order to ensure receiving the Z-count. Further, in order to ensure that random data is not mistaken for the Z-count, two subframes or about twelve seconds, are sometimes observed. However, there are certain GPS positioning systems where it is undesirable or impractical to receive or use a GPS signal burst that is twelve or even six seconds long.
GPS processing techniques have been developed using a relatively accurate, for example 100 milliseconds, knowledge of time as an assumed time for eliminating the requirement for receiving the Z-count. Unfortunately, there are circumstances where these techniques cannot be used because time with this accuracy is not available. For example, a stand-alone event capture device such as a digital camera might have a real time clock that accumulates an error of one second per day or might be incorrectly set by a user for a time error of several hours or even days.